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Effects on sea level measurement of the earthquakes occurring off the coast of Sumatra on 11 April 2012

Following the Mw8.6 and Mw8.2 magnitude earthquakes that struck on 11 April 2012 at 0838 and 1043 UT respectively off the coast of the Indonesian island of Sumatra (the first earthquake occurring at a depth of 23 km and approximately 430 km South-West of Banda Aceh, the capital of the Aceh province), a tsunami alert was initiated.

Location and information concerning the earthquakes off the coast of Sumatra on 12/04/2012Source NOAA

The waves caused by the earthquake did not exceed a metre in height on the West coast of Sumatra.

Click on the link to view the propagation of the tsunami generated by the earthquake on 11/04/2012 off the coast of Sumatra (Photo credits NOAA)

Five hours after the earthquake, the US tsunami alert centre in Hawaii, announced that it had raised the tsunami alert for the whole of the Indian Ocean. "The sea levels observed currently indicate that the threat has reduced for most of the areas concerned", the US scientists justified.

Significant wave height out at sea (that does not reflect the wave height on the coasts). Source NOAA Source NOAA

In fact, by its mechanism this earthquake was not, by nature, tsunami-generating. Christophe Vigny, Deputy Director of the Laboratoire de géologie de l'Ecole normale supérieure de Paris, and Director of Research at the CNRS explains, in the columns of the newspaper Le Monde: "In 2004, the earthquake was produced by a subduction phenomenon: the sliding of the Indian tectonic plate under the Sunda continental plate (sort of Eurasian promontory in South-East Asia). As for the earthquake on 11 April, it occurred within the Indian plate itself. It was the result of a northward horizontal sliding motion along a major vertical fault, angled 20 degrees eastwards".".

In France, the orange alert initiated on the island of Reunion was raised a few hours later. The prefect, Michel Lalande, had alerted the captains of ships alongside and at sea, as well as town halls, emergency centres and the law enforcement agencies of coastal communes. All bathing and walking along the coast was prohibited.

Sea level observations by French tide gauges in the Indian Ocean from 10 to 12 April 2012. In green, French tide gauges broadcasting their data in near real-time; in purple, tide gauge stations not broadcasting in real-time. The tide gauges at Dzaoudzi, Pointe-des-Galets and Sainte-Marie belong to SHOM's RONIM network (Click on the map to enlarge it)

The Sainte-Marie tide gauge and, to a lesser extent, that at the Pointe-des-Galets, both installed on the island Reunion, show that the sea level for both sites was disturbed by the earthquake and the generation of waves affecting the water height (amplitudes of around 20 and 10 cm respectively)..

How the tsunami alert system works. Interview with François Schindelé, expert on tsunami phenomena at the CEA and chairman of the intergovernmental alert system coordination group in the Mediterranean and North Atlantic, by Aude Lasjaunias. Article published in the newspaper Le Monde on 12/04/2012. (fr), par Aude Lasjaunias. Article du Monde, paru le 12/04/2012.